Q: What is the net chemical equation for one turn of the citric acid cycle?

$\text{Acetyl-CoA} + 2\text{H}_2\text{O} + 3\text{NAD}^+ + \text{FAD} + \text{ADP} + \text{P}_i \rightarrow 2\text{CO}_2 + 3\text{NADH} + 3\text{H}^+ + \text{FADH}_2 + \text{ATP} + \text{CoA-SH}$

Q: What is the total ATP yield per mole of glucose oxidized, considering the P/O ratios for NADH and FADH2?

$4 + (2.5 \times 10) + (1.5 \times 2) = 32$ moles of ATP

Question 1

Which of the following statements best describes the role of the citric acid cycle in cellular metabolism?

Accepted Answer

It is the central pathway for oxidizing all metabolic fuels, storing most energy yield in reduced electron carriers such as NADH.

Question 2

During cellular respiration, which stage is responsible for the reoxidation of reduced electron carriers and the majority of ATP synthesis?

Accepted Answer

Stage 3

Question 3

Where do the reactions of stages 1 and 2 of cellular respiration occur in eukaryotic cells?

Accepted Answer

In the mitochondrial matrix

Question 4

Which enzyme complex converts pyruvate to acetyl-CoA, and what are its three component enzymes?

Accepted Answer

Pyruvate dehydrogenase complex; E1: pyruvate dehydrogenase, E2: dihydrolipoamide transacetylase, E3: dihydrolipoamide dehydrogenase

Question 5

Which reaction in the citric acid cycle is highly exergonic due to thioester hydrolysis, allowing the cycle to proceed even at low oxaloacetate concentrations?

Accepted Answer

Citrate synthase

Question 6

What is the net chemical equation for one turn of the citric acid cycle?

Accepted Answer

$	ext{Acetyl-CoA} + 2	ext{H}_2	ext{O} + 3	ext{NAD}^+ + 	ext{FAD} + 	ext{ADP} + 	ext{P}_i ightarrow 2	ext{CO}_2 + 3	ext{NADH} + 3	ext{H}^+ + 	ext{FADH}_2 + 	ext{ATP} + 	ext{CoA-SH}$

Question 7

Which citric acid cycle enzyme is membrane-bound and delivers electrons directly into the mitochondrial electron transport system via coenzyme Q?

Accepted Answer

Succinate dehydrogenase

Question 8

What is the main function of substrate-level phosphorylation in the citric acid cycle, and which enzyme catalyzes this reaction?

Accepted Answer

Direct synthesis of ATP or GTP from ADP or GDP; catalyzed by succinyl-CoA synthetase

Question 9

Despite having a large positive standard free energy change, why does the malate dehydrogenase reaction proceed in vivo?

Accepted Answer

Because the highly exergonic citrate synthase reaction keeps oxaloacetate levels very low

Question 10

Which electron carrier in the mitochondrial respiratory chain can transfer two electrons in one step and provides a link between two-electron and one-electron carriers?

Accepted Answer

Coenzyme Q (ubiquinone)

Question 11

What is the chemiosmotic coupling model proposed by Mitchell in 1961?

Accepted Answer

Electron transport drives the active transport of protons from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient.

Question 12

How many NADH and FADH2 molecules are produced per mole of glucose during stages 1 and 2 of cellular respiration (including glycolysis and the citric acid cycle)?

Accepted Answer

10 NADH and 2 FADH2

Question 13

What is the total ATP yield per mole of glucose oxidized, considering the P/O ratios for NADH and FADH2?

Accepted Answer

$4 + (2.5 	imes 10) + (1.5 	imes 2) = 32$ moles of ATP

Question 14

Which of the following best explains why succinate is not highlighted after step 5 in the fate of carbon diagram for the citric acid cycle?

Accepted Answer

Succinate is symmetrical, so the origin of its carbon atoms cannot be distinguished.

Question 15

Which of the following electron carriers in the respiratory chain are single electron carriers and consist of nonheme iron complexed with thiol sulfurs of cysteine residues?

Accepted Answer

Iron-sulfur clusters

Question 16

Which citric acid cycle enzyme catalyzes a highly stereospecific hydration reaction, attacking only the trans double bond to form L-malate?

Accepted Answer

Fumarase

Question 17

Which reaction in the citric acid cycle is analogous to the pyruvate dehydrogenase complex and conserves the energy of oxidation as NADH?

Accepted Answer

Alpha-ketoglutarate dehydrogenase complex

Question 18

In the context of mitochondrial structure, which compartment contains the enzymes for the citric acid cycle and pyruvate oxidation?

Accepted Answer

Mitochondrial matrix

Question 19

Which of the following best describes the flow of electrons in the mitochondrial respiratory chain?

Accepted Answer

From low reduction potential carriers to high reduction potential carriers

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